1. Field of the Invention
The present invention relates to improvements in scanning apparatus and more particularly to electro-optical scanning apparatus which is self-compensated for variations in reflectivity.
2. Description of the Prior Art
In automatic character recognition systems the typical devices include a transported scanner which is passed across a printed page and which, during passing, detects local changes of reflectivity in the printed page to identify the various line forms making up each character. Basic to any such character recognition system therefore is the ability of the scanner to detect the presence of various lines on a piece of paper. Discrimination of lines entails processing of a plurality of variables such as the reflectivity of the paper on which the line is made, the thickness of the line and the strength or contrast of the line. Since any pattern recognition system designed to recognize various characters must not only discriminate the character pattern but must also, based on that discrimination, select recognition characteristics, any degradation or losses of the line form detail has a direct effect on both the accuracy and the fidelity of the pattern recognition system. Accordingly, the prior requirement to any attempts at pattern recognition is the design of a line scanner which is inherently accurate and which is operative for various levels of impression density or symbol distinction. These latter features of a character line are commonly referred to in the art as character contrast.
In the prior art, many threshold techniques have been developed which are particularly adapted to the varying contrast problem in character recognition. Typical of such threshold systems is the system described in U.S. Pat. No. 3,479,642 to M. R. Bartz. This reference properly recognizes the problem of recognition of a line where the line varies both with the typed font and with the life of the typewriter ribbon. This reference utilizes as the threshold generator the generator described in U.S. Pat. No. 2,975,371 and based on that threshold generates line width by counting up the number of black, or below threshold reflectivity, signals picked up by the scanner. The line width is then compared against a typical or average line width and converted to an analog value. Alternatively, variable gain or gain compensated techniques, such as the technique described in U.S. Pat. No. 3,714,397, have been developed. In each case, an average threshold level of reflectivity within the aperture is developed against which changes due to symbol presence are compared, again on an averaged basis, such that the minimum detail recognized is controlled by the dimensions of the scanning aperture.